The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device where liquid crystal molecules are aligned axially symmetrically in liquid crystal regions separated from one another by polymer walls.
Conventionally, twisted nematic (TN) and super-twisted nematic (STN) liquid crystal display devices (LCDS) are used as display devices utilizing the electrooptic effect. In order to widen the viewing angle of these LCDs, various techniques are now under vigorous development.
As one of the viewing angle widening techniques proposed so far, Japanese Laid-Open Patent Publication No. 6-301015 and No. 7-120728 disclose a so-called axially symmetrically aligned microcell (ASM) mode LCD where liquid crystal molecules are aligned axially symmetrically in respective liquid crystal regions separated from one another by polymer walls. Each liquid crystal region substantially surrounded by the polymer walls is typically formed for each pixel. Since the liquid crystal molecules are aligned axially symmetrically, such an ASM mode LCD provides a wide viewing angle characteristic where change in a contrast ratio is small whichever direction an observer views the LCD.
The ASM mode LCDs disclosed in the above publications are fabricated by subjecting a mixture of a polymeric material and a liquid crystal material to polymerization induced phase separation.
A method for fabricating a conventional ASM mode LCD will be described with reference to FIG. 7. First, in step (a) shown in FIG. 7, a glass substrate 908 having a color filter and an electrode formed on one surface is prepared. The color filter and the electrode formed on the glass substrate 908 are not shown in FIG. 7 for simplification. Formation of the color filter is described hereinafter.
In step (b), polymer walls 917 for axially symmetrical alignment of liquid crystal molecules are formed in a lattice shape, for example, on the surface of the glass substrate 908 on which the color filter and the electrode are formed. Specifically, a photosensitive resin material is spin-coated on the glass substrate 908, exposed to light via a photomask having a predetermined pattern, and developed to form lattice-shaped polymer walls. The photosensitive resin material may be negative or positive. A non-photosensitive resin material may also be used although a step of forming a resist film is additionally required. An opaque material is used for formation of the polymer walls.
In step (c), column protrusions 920 are formed on the top faces of the resultant polymer walls 917 by dispersive patterning. The column protrusions 920 are obtained by exposing to light and developing a photosensitive resin material as in the polymer walls.
In step (d), the surface of the glass substrate 908 with the polymer walls 917 and the column protrusions 920 formed thereon is coated with a vertical alignment material 921 such as polyimide. Meanwhile, in steps (e) and (f), the surface of a counter substrate 902 on which an electrode is formed is also coated with the vertical alignment material 921.
In step (g), the resultant two substrates are bonded together with the surfaces thereof on which the electrode is formed facing each other, to form a liquid crystal cell. The gap between the two substrates (cell gap, i.e., the thickness of a liquid crystal layer) is defined by the sum of the height of the polymer walls 917 and the height of the column protrusions 920.
In step (h), a liquid crystal material is injected into the resultant cell gap by a vacuum injection method or the like. Finally, in step (i), a voltage is applied between a pair of opposing electrodes to align liquid crystal molecules axially symmetrically in each liquid crystal region 916. That is, liquid crystal molecules in the liquid crystal region 916 defined by the polymer walls 917 are aligned axially symmetrically with respect to an axis 918 (vertical to the substrates) shown by the dashed line in FIG. 7.
FIG. 8 shows a cross-sectional structure of a conventional color filter. The color filter includes colored resin sections of red (R), green (G), and blue (B) corresponding to respective pixels and a black matrix (BM) film for light-shading the gaps between the colored resin sections, which are formed on a glass substrate. The colored resin sections and the BM film are covered with an overcoat (OC) layer made of an acrylic resin or an epoxy resin having a thickness of about 0.5 to 2.0 xcexcm for improving the smoothness and the like. The OC layer is then covered with an indium tin oxide (ITO) film as a transparent signal electrode. The BM film is generally made of a metal chromium film having a thickness of about 100 to 150 nm. The colored resin sections are made of resin materials colored with a dye or a pigment and generally have a thickness of about 1 to 3 xcexcm.
The color filter is formed by patterning photosensitive colored resin layers formed on the substrate by photolithography. For example, R, G, and B photosensitive resin materials are individually subjected to the process of layer formation, light exposure, and development (the process is done total three times), to form the R, G, and B color filter sections. Each photosensitive colored resin layer can be formed by applying a liquid photosensitive colored resin material (obtained by diluting the material with a solvent) to the substrate by spin coating or the like, by transferring a dry film of a photosensitive colored resin material, or other methods. Using the thus-formed color filter, the ASM mode color LCD described above having a wide viewing angle characteristic is obtained.
However, the above conventional ASM mode LCD has the following problems. Although this LCD provides a wide viewing angle characteristic, the transmittance of the LCD lowers because the existence of the polymer walls reduces light transmission and liquid crystal molecules present above the polymer walls do not contribute to display. Moreover, the axially symmetrical alignment of liquid crystal molecules near the polymer walls is disturbed. This may generate light leakage in the black display state, for example, resulting in generation of flicker in an image.
In order to solve the above problems, one of the present inventors developed with co-researchers the following techniques: a technique for improving the transmittance of an LCD by forming the polymer walls using a transparent resin so that liquid crystal molecules present above the polymer walls are contributable to display; and a technique for stabilizing the axially symmetrical alignment of liquid crystal molecules present near the polymer walls by tilting the side faces of the polymer walls with respect to the substrate surface (Japanese Patent Application No. 10-185495).
The LCD disclosed in the above application exhibited improved contribution to display of the liquid crystal molecules present above the polymer walls, compared with the conventional LCD using an opaque resin material. However, the alignment of these liquid crystal molecules present above the polymer walls is disturbed. As a result, roughness (local variation in contrast ratio) is sometimes observed in the display image.
An object of the present invention is providing a liquid crystal display device that has a wide viewing angle characteristic and can realize bright and roughness-free display.
The liquid crystal display device of the present invention includes a first substrate, a second substrate, and a liquid crystal layer sandwiched by the first and second substrates, wherein the first substrate includes polymer walls made of a transparent resin, the liquid crystal layer has a plurality of liquid crystal regions separated from one another by the polymer walls, and liquid crystal molecules in the plurality of liquid crystal regions are aligned axially symmetrically with respect to respective axes formed in the plurality of liquid crystal regions, the axes being vertical to a surface of the first substrate, and liquid crystal molecules above the polymer walls are aligned axially symmetrically with respect to respective axes formed on the polymer walls, the axes being vertical to the surface of the first substrate.
The width of the polymer walls is preferably three sevenths or less of the width of the liquid crystal regions adjacent to the respective polymer walls.
The width of the liquid crystal regions is preferably 150 xcexcm or less.
The polymer walls preferably have a face tilting with respect to the surface of the first substrate.
Preferably, the tilt face of the polymer walls has a first tilt portion tilting at a first tilt angle with respect to the surface of the first substrate and a second tilt portion tilting at a second tilt angle with respect to the surface of the first substrate, the first tilt angle is smaller than the second tilt angle, and the first tilt portion is closer to the surface of the first substrate than the second tilt portion.
The first tilt angle is preferably 5xc2x0 or less, more preferably in the range between 3xc2x0 and 5xc2x0, inclusive. The second tilt angle is preferably 10xc2x0 or more, more preferably in the range between 10xc2x0 and 90xc2x0, inclusive.
The liquid crystal layer may include a liquid crystal material having negative dielectric anisotropy.
The height of the polymer walls is preferably smaller than a half of the thickness of the liquid crystal layer.